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Oceans have absorbed up to 30 percent of human-made carbon dioxide around the world, storing dissolved carbon for hundreds of years. As the uptake of carbon dioxide has increased in the last century, so has the acidity of oceans worldwide. Since pre-industrial times, the pH of the oceans has dropped from an average of 8.2 to 8.1 today. Projections of climate change estimate that by the year 2100, this number will drop further, to around 7.8 — significantly lower than any levels seen in open ocean marine communities today.

Now a team of researchers from MIT, the University of Alabama at Birmingham, and elsewhere has found that such increased ocean acidification will dramatically affect global populations of phytoplankton — microorganisms on the ocean surface that make up the base of the marine food chain.

In a study published today in the journal Nature Climate Change, the researchers report that increased ocean acidification by 2100 will spur a range of responses in phytoplankton: Some species will die out, while others will flourish, changing the balance of plankton species around the world.

The researchers also compared phytoplankton’s response not only to ocean acidification, but also to other projected drivers of climate change, such as warming temperatures and lower nutrient supplies. For instance, the team used a numerical model to see how phytoplankton as a whole will migrate significantly, with most populations shifting toward the poles as the planet warms. Based on global simulations, however, they found the most dramatic effects stemmed from ocean acidification.

Stephanie Dutkiewicz, a principal research scientist in MIT’s Center for Global Change Science, says that while scientists have suspected ocean acidification might affect marine populations, the group’s results suggest a much larger upheaval of phytoplankton — and therefore probably the species that feed on them — than previously estimated.

“I’ve always been a total believer in climate change, and I try not to be an alarmist, because it’s not good for anyone,” says Dutkiewicz, who is the paper’s lead author. “But I was actually quite shocked by the results. The fact that there are so many different possible changes, that different phytoplankton respond differently, means there might be some quite traumatic changes in the communities over the course of the 21st century. A whole rearrangement of the communities means something to both the food web further up, but also for things like cycling of carbon.”

The paper’s co-authors include Mick Follows, an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

Winners and losers

To get a sense for how individual species of phytoplankton react to a more acidic environment, the team performed a meta-analysis, compiling data from 49 papers in which others have studied how single species grow at lower pH levels. Such experiments typically involve placing organisms in a flask and recording their biomass in solutions of varying acidity.

In all, the papers examined 154 experiments of phytoplankton. The researchers divided the species into six general, functional groups, including diatoms, Prochlorococcus, and coccolithophores, then charted the growth rates under more acidic conditions. They found a whole range of responses to increasing acidity, even within functional groups, with some “winners” that grew faster than normal, while other “losers” died out.

The experimental data largely reflected individual species’ response in a controlled laboratory environment. The researchers then worked the experimental data into a global ocean circulation model to see how multiple species, competing with each other, responded to rising acidity levels.

The researchers paired MIT’s global circulation model — which simulates physical phenomena such as ocean currents, temperatures, and salinity — with an ecosystem model that simulates the behavior of 96 species of phytoplankton. As with the experimental data, the researchers grouped the 96 species into six functional groups, then assigned each group a range of responses to ocean acidification, based on the ranges observed in the experiments.

Natural competition off balance

After running the global simulation several times with different combinations of responses for the 96 species, the researchers observed that as ocean acidification prompted some species to grow faster, and others slower, it also changed the natural competition between species.

“Normally, over evolutionary time, things come to a stable point where multiple species can live together,” Dutkiewicz says. “But if one of them gets a boost, even though the other might get a boost, but not as big, it might get outcompeted. So you might get whole species just disappearing because responses are slightly different.”

Dutkiewicz says shifting competition at the plankton level may have big ramifications further up in the food chain.

“Generally, a polar bear eats things that start feeding on a diatom, and is probably not fed by something that feeds on Prochlorococcus, for example,” Dutkiewicz says. “The whole food chain is going to be different.”

By 2100, the local composition of the oceans may also look very different due to warming water: The model predicts that many phytoplankton species will move toward the poles. That means that in New England, for instance, marine communities may look very different in the next century.

“If you went to Boston Harbor and pulled up a cup of water and looked under a microscope, you’d see very different species later on,” Dutkiewicz says. “By 2100, you’d see ones that were living maybe closer to North Carolina now, up near Boston.”

Dutkiewicz says the model gives a broad-brush picture of how ocean acidification may change the marine world. To get a more accurate picture, she says, more experiments are needed, involving multiple species to encourage competition in a natural environment.

“Bottom line is, we need to know how competition is important as oceans become more acidic,” she says.

This research was funded in part by the National Science Foundation, and the Gordon and Betty Moore Foundation.

Comments

Freelance Philosophy

July 20, 2015

In addition to competition, the incorporation of some variability in fitness (mutation) in reproductive communities would be interesting as species under high selective pressures get a potential "out" from their circumstance. Such fitnesses best understood by competitive cell culture in labs. I can imagine pretty pictures here.

nealjking

July 21, 2015

I can imagine some not-so-prettty pictures in the real world.

Bob Bingham

July 23, 2015

Ocean acidity is the biggest threat to civilisation. We need to stop burning fossil fuels fast.

zackjacobson

July 24, 2015

worst news I can think of.

Tin Cup NYC

July 25, 2015

When I read these things are taking place, I realize that we don't have leaders capable of tackling the more serious environmental problems. As Fay Gillis Wells once said to me: "Those of us who know better must do more."

Cichlid61

July 30, 2015

Don't phytoplankton, as a whole, also produce a fairly significant percentage of our oxygen? iirc, it was something comparable to the Amazon's rainforest.

Sam Makco

August 6, 2015

Our main problem today is that, for far too long, we have elected politicians for their 'ability' to SOLVE problems caused by their predecessors, instead of electing politicians for their ability to AVOID problems in the first place!

one percenter

August 10, 2015

Why is the author of this study cushioning his words? Say it. Climate change will wipe out most plankton which account for 50% of all oxygen we breathe and absorb 30% of carbon dioxide. This will dramatically accelerate ocean acidification and climate change while dramatically reducing global oxygen levels...

If plankton species die out that will reduce the amount of dissolved oxygen in the oceans. Combined with the hotter acidic water..huh, not good. Think ocean dead zones that reach from one continental coast to the next. We are going to replace plankton with highly toxic algae that emit hydrogen sulfide.

There is evidence this EXACT scenario (plankton die out due to acidification caused by methane and carbon, creating hypoxic oceans that burp cataclysmic levels of hydrogen sulfide and suffocate 95% of all life on the planet) happened in the 6th great extinction.

We already have definitive evidence that whatever plankton do survive they are in apparently much lower numbers. We've already had a 40% decline in plankton since the 1950s (think about that for a sec). What this study is predicting is the EXTINCTION of plankton species soon. Maybe this is why all the marine animals (from anchovies to whales) are now clustered very very close to the Pacific coast (last month over a 115 whales spotted at one time).

If algae cannot survive the open ocean shouldn't you guys be called for us to build domes for our cities or calling for a ban on all new gas-powered cars within a year. I'm simply paraphrasing all these studies. Remember this post next time you see a Koch bros commercial.